I am just after some ideas for what might be causing my recent rough running on my 912 ULS.   The engine is in a homebuilt and both engine and airframe have done about 400 hours.   Recently the engine has been running a bit rough and it seems to be exclusively on hot days.  Originally I thought it was an issue with the carbs so I tore them down a few times and never found an issue with them.   After this continued to happen a couple more times I decided to just bite the bullet and purchased 2 brand new carbs.   The engine didn’t have any more issues after this and I thought the problem was solved, but the weather more recently has been a bit cooler.  Last week it was hot again and the issue came up again after my takeoff climb.   The rough running wasn’t that alarming and was similar to if you turned off a mag bank with the engine on idle….a bit rough and some loss of power but it didn’t seem like it was going to quit or anything.   The gauges were all still fine but since it seemed to only happen on hot days I dropped the oil cooler down a bit to get some cooler air from under the cowling and also replaced the spark plugs in case the old carbs caused fouling of any of them.   Today I flew it again.   It was a hot day, 35C/ 95F so I did an enroute climb at 85kts/1000fpm instead of my normal 70kts/1500fpm.   Despite this the engine started to seem a little rough by about 1500 feet so lowered the nose further and powered back a bit.   This wasn’t enough to fix the issue and the engine got so rough that I actually thought it was going to quit this time.   It was doing about 3800RPM on full power.   I pulled the power right back to idle and it smoothed out, and brought it back to the field without powering back up.   By the time I was on the ground it had cooled off a bit and did a ground run-up without any issues.   Obviously I haven’t fixed the problem so wanted to see if any of you had any ideas.   Some relevant info:

The engine has run hot many times in the past while test flying the plane as I was sorting out the cowling, air intakes, radiator size etc.   During test flights I was often on the upper limits of CHT and oil temperatures but the engine never missed a beat.   The engine is running much cooler now yet this issue is new.

The issue seems to be exclusively on hot days, after high power use such as takeoff climb.   Despite it seeming to be directly related to heat the gauges don’t show any significant increase in temperatures compared to other days when I don’t have issues.   The cowl is tight and the engine does normally run a bit hot.   CHT is often about 110C/ 230F on takeoff so I use waterless coolant.   This has never been an issue until now, and on these hot days the temperatures are only up a few degrees from this. 

The oil pressure and fuel pressure remain normal at all times.   The carbs have a couple hours on them since new and have not been adjusted from factory settings.  Plugs today were brand new but the ones I took out did not look fouled in any way.  

Previously when the engine has just run a bit rough but not alarmingly, I was able to power back for a few minutes and let the engine cool a bit and then it would power back up without issue.   I didn’t attempt this today due to the severity of the rough running. 

Any ideas would be helpful and appreciated.   I can cut up the cowling and get the CHTs and oil temps down at the expense of some cruise speed if I need to, but I would still like to know what is causing this when it was running fine before with the same set-up.   Thanks!

  • Re: rough running 912ULS

    by » one year ago

    Does your fuel system have a "return line" between the "T-fitting" where the supply line spits to go to the two carburetors, so excess fuel can return to the fuel tank (or header tank, as the case may be)?  If not, you could be encountering some fuel vaporization in the lines due to higher temps. Having that return line is recommended by Rotax, and I've seen several posts where folks experiencing "vapor bubbles" in the fuel resolved the issue by adding the return line.  Perhaps one of the "gurus" can explain it better than I have...

  • Re: rough running 912ULS

    by » one year ago

    Bret, sounds like you have a good grasp on things.  I'm nowhere near the expert you are but I was thinking venting or fuel return possibly.  Since it's a temp/heat issue, you might call the rotax places in hot regions like the one in Mississippi or Florida.  I'll be watching.

    One last stupid question - of course the first thing we think of with rough running is a carb sync issue.  But you said you did that.  Is it possible it's a cable issue - stretched or something?  So while on the ground the sync issue performs fine, but then maybe airborne, maybe both carbs don't open the exact same?

    I'll shut up now.


  • Re: rough running 912ULS

    by » one year ago

    Hi Guys thanks for the input.   I have the recommended set-up where after the mechanical pump the line splits off to both carbs and also back to the gascolator through a 0.02" restrictor.   This has given me a fairly reliable 3-4 PSI fuel pressure and I checked to make sure the return line was patent recently.   

    The carbs are synched and to avoid issues such as the one you mentioned I used music wire for my throttles so they are a rigid push/pull setup and shouldn't have any ability to flex or stretch.   With how rough the engine was it would have needed to have one carb almost completely closed with the other open so it is definitely not flexing that much.   

    So far there seems to be a consensus the issue is still likely fuel/carb related.  I would have to agree the heat causing vapour or mixture related issues makes more sense than that the oil or CHTs being hot would cause such a massive issue in itself.   It is technically possible to have adequate fuel pressure but not adequate flow, so what I might do as a next measure is take off the fire covers on all the hoses to make sure none of them are starting to kink.   They all look fine with the fire hoses on but if one has a bit of a kink when cold then when it gets hot it could be severely restricting.   I will also check the filters off the tanks to make sure they are not getting packed in (I replace them on my 100 hourly either way so they should be fine).   If it is a vapour issue that for some reason has newly come up then I could also fill one tank with 100LL and see if the problem goes away or gets better running Avgas and if it does than at least I know what system to be focusing on.   

  • Re: rough running 912ULS

    by » one year ago

    "The carbs are synched and to avoid issues such as the one you mentioned I used music wire for my throttles so they are a rigid push/pull setup and shouldn't have any ability to flex or stretch.   With how rough the engine was it would have needed to have one carb almost completely closed with the other open so it is definitely not flexing that much." 


    One more thing to rule in or out.

    The issue may be with your throttle cable. I would put a set of carb sync gauges on the engine and run it from idle to around 3300 - 3500 rpm and see if the gauges read the same on each and every run up. I have seen may throttle systems that do not go back to the same place each time the throttle is advanced and it makes trying the sync the carbs a pain and sometimes you just have to  live with those results. I have found thinner cabling to be smoother and work better than thick heavy or ridged cabling.  The thinner cable slides easily and smoothly through the throttle cable sheath and allows the carb spring to do its job. Those carbs are easy to sync and stay that way every run up of the rpm. 

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

  • Re: rough running 912ULS

    by » 12 months ago


    You say that you are returning fuel to the gascolator.  If that is in fact your set up, you are simply recirculating hot fuel.  The fuel return line should return fuel to a fuel tank, where the heat it picked up from the engine dissipates.

    Fuel needs to vaporize in order to burn properly, but it should vaporize in the combustion chamber/intake manifold, and not before.  Fuel boiling in the lines can cause vapor locks, and fuel boiling in the float bowls can cause flooding.

    If you don't have one, install a fuel return line from the fuel distributor back to the fuel tank.

    Richard W


    Thank you said by: RotaxOwner Admin

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